The present invention relates to a semiconductor device. More particularly, the present invention relates to a technology effectively applicable to a semiconductor device designed into a configuration of a multistage amplifier circuit.
A semiconductor device known as a high-frequency power amplifier (or a high-frequency power module) is incorporated in a portable communication apparatus such as a portable telephone or a car telephone of the PDC (Personal Digital Cellular) system or a portable telephone of the PHS (Personal Handyphone System). This high-frequency power amplifier is designed into a configuration of a multistage amplifier circuit in which a plurality of amplifying means are electrically connected to each other to form a multistage structure.
The high-frequency power amplifier is built by mounting a semiconductor chip on a main surface of a wiring substrate. The semiconductor chip has an amplifying means formed on a main surface thereof. Electrodes formed on a main surface of the semiconductor chip are electrically connected to electrodes formed on a main surface of the wiring substrate by conductive wires. The amplifying means has a configuration in which typically a plurality of field-effect transistors are electrically connected to each other to form a parallel circuit. A gate terminal (serving as the input unit) of the amplifying means is electrically connected to a chip-side input electrode formed on the main surface of the semiconductor chip. On the other hand, a drain terminal (serving as the output unit) of the amplifying means is electrically connected to a chip-side output electrode formed on the main surface of the semiconductor chip. The chip-side input electrode is placed at a position on a particular side of the semiconductor chip whereas the chip-side output electrode is placed at a position on another side of the semiconductor chip facing the particular side. A source terminal of the amplifying means is electrically connected to a back-surface electrode formed on a back surface of another semiconductor chip facing the main surface. The back-surface electrode is fixed at a reference electric potential. The chip-side input electrode is electrically connected to a substrate-side input electrode formed on the main surface of the wiring substrate by an input wire. The substrate-side input electrode is placed at a position facing the particular side of the semiconductor chip cited above. The chip-side output electrode is electrically connected to a substrate-side output electrode formed on the main surface of the wiring substrate by an output wire. The substrate-side output electrode is placed at a position facing the other side of the semiconductor chip cited above.
By the way, in order to reduce the size and the cost of the high-frequency power amplifier, an attempt has been made to form a plurality of amplifying means on one semiconductor chip. In the case of two amplifying means formed on one semiconductor chip, for example, the amplifying means at the front stage is oriented in a direction opposite to a direction in which the amplifying means at the rear stage is oriented so that the input and the output of the amplifying means at the front stage are placed at locations in close proximity to respectively the output and the input of the amplifying means at the rear stage. As a result, the input and output wires at the front stage and the output and input wires at the rear stage are close to each other. As a result, there is raised a problem of a deteriorating high-frequency characteristic due to a mutual-induction effect between the input and output wires. In particular, the mutual-induction effect between the input wire of the front stage and the output wire of the rear stage is a serious problem since a difference between a power flowing through the input wire and a power flowing through the output wire is big.
A technology to prevent the high-frequency characteristic from deteriorating due to a mutual-induction effect between wires is disclosed for example in Japanese Patent Laid-open No. Hei 9-260412 (1997). According to this technology, a chip-side bonding electrode is formed between the chip-side input electrode and the chip-side output electrode whereas a substrate-side bonding electrode is formed between the substrate-side input electrode and the substrate-side output electrode. The chip-side bonding electrode is electrically connected to the substrate-side bonding electrode and, by fixing the chip-side bonding electrode and the substrate-side bonding electrode at a reference electric potential, the high-frequency characteristic can be prevented from deteriorating due to a mutual-induction effect between the input and output wires.
In addition, the high-frequency power amplifier module employing transistors is a key device of a portable telephone of mobile communication adopting systems such as the PDC (Personal Digital Cellular) system and the GSM (Global System for Mobile communication). The demand for such a portable telephone has been growing tremendously in recent years. Specifications of such a high-frequency power amplifier include a small size and a low cost in addition to good high-frequency characteristics for applications to mobile communication systems.
A technique to respond to such a demand is disclosed in Japanese Patent Laid-open No. 2755250. By placing 2 transistors, namely, a first-stage transistor 2000 and a second-stage transistor 3000, at locations close to each other on a semiconductor chip 1000 as shown in a top-view diagram of FIG. 21 and a squint-view diagram of FIG. 22, the size and the cost can be reduced. A bonding input electrode 2000b of the first-stage transistor 2000 is electrically connected to a bonding electrode 7000d of a wiring substrate 6000 by an input bonding wire 9000d. A bonding output electrode 3000c of the second-stage transistor 3000 is electrically connected to a bonding electrode 7000a of the wiring substrate 6000 by an output bonding wire 9000a. A bonding electrode 10000a on the semiconductor chip 1000 is electrically connected to a bonding electrode 12000a of the wiring substrate 6000 by a shield bonding wire 13000a. The shield bonding wire 13000a is provided between the input bonding wire 9000d and the output bonding wire 9000a. The bonding electrode 10000a and the bonding electrode 12000a at the ends of the shield bonding wire 13000a are connected to the ground at high frequencies by via holes bored through the semiconductor chip 1000 and the wiring substrate. It should be noted that the via holes themselves are not shown in the figure. By providing a shield bonding wire 13000a, the amount of coupling through a mutual inductance between the input bonding wire 9000d and the output bonding wire 9000a can be reduced, allowing the degree of deterioration of isolation between the high-frequency input and output terminals to be lowered. As a result, the high-frequency characteristic is improved.
The problem of coupling through a mutual inductance between the input bonding wire 9000d and the output bonding wire 9000a is raised by a location of the input of the first-stage transistor 2000 in close proximity to a location of the output of the second-stage transistor 3000 and a location of the output of the first-stage transistor 2000 in close proximity to the location of the input of the second-stage transistor 3000 which are caused by the fact that the first-stage transistor 2000 and the second-stage transistor 3000 are oriented in directions opposite to each other. In particular, the mutual-induction effect between the input bonding wire 9000d of the first-stage transistor 2000 and the output bonding wire 9000a of the second-stage transistor 3000 is a serious problem. This is because the high-frequency power output by the second-stage transistor 3000 is higher than the high-frequency power input to the first-stage transistor 2000 by 20 to 30 dB (or 100 to 1,000 times), giving rise to a positive feedback from the output to the input. Even though the output bonding wire 9000c of the first-stage transistor 2000 and the input bonding wire 9000b of the second-stage transistor 3000 are also close to each other, the problem of a deteriorating high-frequency characteristic caused by a mutual-induction effect does not arise due to the fact that a ratio of a high-frequency power flowing through the input bonding wire 9000b to a high-frequency power flowing through the output bonding wire 9000c is not greater than 0 dB (1 time).
In FIGS. 21 and 22, reference numerals 2000a and 3000a denote the main bodies of the first-stage transistor 2000 and the second-stage transistor 3000 respectively. Reference numerals 2000d and 3000d denote the source electrodes of the first-stage transistor 2000 and the second-stage transistor 3000 respectively. Reference numeral 2000c denotes the bonding output electrode of the first-stage transistor 2000 and reference numeral 3000b denotes the bonding input electrode of the second-stage transistor 3000. Reference numeral 4000 denotes a ground electrode whereas reference numerals 7000b and 7000c each denote a bonding electrode of the wiring substrate 6000. Reference numerals 8000a to 8000d each denote a lead electrode and reference numeral 104 denotes a cavity.